In this connection the invention shall include disc brakes which either comprise a sliding caliper or a fixed caliper, which overlap one or more brake discs. Mainly but not exclusively the invention relates to disc brakes with lining sections of the spot-type.
Disc brakes, in particular for heavy load trucks, are known with different configurations, both with respect to the type of the actuation mechanism, with respect to the way of the transmission of the braking or clamping force onto one or several brake discs and with respect to the type of the adjustment for compensating the wear of the brake pad and/or the brake disc as well as in this context with respect to the type of the adjustment of the running clearance.
Generally, there exists a wide range of actuation mechanism and brake adjuster designs applied and embodied in disc brakes.
From EP 0 271 864 B1 an actuation device is known which comprises an axial actuation member around which several components are arranged such as a force amplification mechanism in the form of a roller-ramp mechanism. Commonly used actuation mechanisms do embody brake levers which are actuated by hydraulic, pneumatic or electro-mechanic actuators, which lever is transmitting the introductory force usually by means of one or more roller elements to a thrust piece, which is mainly axially guided in the housing of the brake caliper towards the brake disc. Such lever-actuated brake mechanisms are e.g. known from EP 0 553 105 B1 or EP 0 698 749 B1.
An alternative design of a brake actuation mechanism is shown in WO 2001/75324 A1, in which the thrust element for transferring the clamping force onto the brake disc is arranged around a central rod. A similar design is e.g. known from WO 2004/059187 A1 of the applicant. The brake actuation mechanism for a disc brake as disclosed therein comprises the shape of one single tappet and thrust element, respectively, which acts onto the brake disc, in which an adjustment device is already integrated within the tappet. The tappet thereby is mounted in the housing of the brake caliper in parallel to the rotary axis of the brake disc by means of a rod. According to a preferred embodiment the tappet directly cooperates with a return mechanism for the thrust element, which is integrated in the tappet and which cooperates with the rod.
Independent from the design of the actuation mechanism to be employed in this connection, a compensation of the running clearance between the brake disc and the brake lining at the brake pad, which results from the wear at the brake pad lining and/or at the brake disc, has to be always provided. The brake actuation mechanism according to this example of a tappet as mentioned herein at least comprises a thrust element which transmits the clamping force onto the brake disc by means of the brake pads. For that purpose the thrust element comprises at least one rotatable element which interacts with a further element, so that both elements can be axially displaced in relation to each other, in which the latter element is guided in the brake caliper in a non-rotatable manner, however axially displaceable.
As an example such principle is realized by WO 2004/059187 A1 in which two sleeve-like spindles are in a threaded engagement, in which an external sleeve having an internal thread receives an internal sleeve having an external thread in a rotational manner. The external sleeve or outer spindle is supported in the brake caliper or in its carrier in a non-rotatable way, so that rotation of the internal sleeve or inner spindle results in a linear displacement of the outer spindle in relation thereto, so that the outer spindle can be moved towards the brake disc in order to compensate the wear-induced clearance at the brake linings.
For example, the non-rotatable linear guidance of the outer spindle in axial direction can be realized in that the outer spindle is connected either directly or by means of further intermediate elements with the brake pad or the brake lining retainer, which in turn itself are guided in the brake caliper or in the caliper carrier in a linear manner. In other words, due to the fact that the brake pad (brake lining and lining retainer), the lining retainer or the brake lining themselves are guided in the caliper or its carrier in axial direction relative to the brake disc without being rotatable, also the outer spindle of the adjuster being connected with these brake components is exclusively axially and non-rotatably guided.
From WO 2011/113554 A2 of the applicant an enhanced single tappet-type actuation mechanism is known, in which the elements of the return mechanism and the adjustment mechanism are incorporated into the tappet design and surrounded by the unit consisting of the outer and inner sleeves, which both form part of the adjustment mechanism and thus brake actuation mechanism. The adjustment mechanism incorporated therein is driven by the rotatable lever, which introduces the clamping force into the brake actuation mechanism. The clearance between the brake pad and the brake disc according to the actual wear is determined in that the rapid change in the clamping force at brake pad to brake disc contact is sensed merely by mechanical means, so that the adjustment mechanism of that type is automatically adjusting the clearance in a mechanical manner only.
Since the adjustment mechanism known from WO 2011/113554 A2 comprises at least one threaded adjustment screw (inner spindle) interacting with at least one other threaded part (outer spindle) for transmitting the actuating force to the inner brake pad, upon brake application a controlled adjustment is performed in the direction of clearance reduction only. After passing a predetermined distance corresponding to a target clearance between the brake pad and the brake disc, a torque is applied to the threaded parts, when the brake disc comes into contact with the brake pads, thereby forming a closed force flow within the brake actuation mechanism and the caliper. This torque is transmitted from the rotatable lever over an one-way clutch in its driving direction and limited by a torque limiting clutch slipping or overrunning when the rotational resistance between the threaded parts rises due to the axial force, which is built up when the brake pad contacts the brake disc, thereby creating some kind of counter-torque.
In summary, the principle and functioning of such automatically acting adjusters, commonly referred to as “automatic brake adjusters” in the field of disc brakes, for the brake actuation mechanisms of the prior art as mentioned above is more or less the same. By means of at least one clutch, which comprises some degree of lost motion, in which the level of free motion is selected to be equivalent to the desired target clearance, i.e. the maximum allowable running clearance, the friction lining of the brake pad, e.g. the brake pad itself, will be progressively advanced towards the brake disc as the friction lining slowly decreases due to wear. Namely, if the friction linings do wear to such an extent that the running clearance is greater than the target clearance, the existing running clearance between the brake pad and the brake disc is taken up and the further additional movement of the brake pad towards the brake disc, which results from and thus corresponds to the actually existing excessive lining wear, causes the clutch to rotate, which rotation will advance the brake pad towards the brake disc. After contact of the brake pad with the brake disc, the increase load in the entire brake mechanism will cause the clutch to slip, thereby preventing further, now not required adjustment movements or any overloading of the adjustment mechanism.
In this respect it has been important that for all prior art brake actuation mechanisms the target clearance between the brake disc and the brake pad is set to be rather large in order to provide enough room for any thermal expansions of either the brake disc or the brake pad resulting e.g. from excessive braking. This should avoid the risk of force contact between these components in the release state of the disc brake, since such force contact would create an energy consuming dragging and may produce even more thermal expansion.
The available stroke of the actuator is generally selected to adapt to any elastic tensioning or widening of the brake caliper or other parts of the disc brake which will be loaded by the clamping force as these components are arranged within the closed flow of forces upon brake actuation. Furthermore, this stroke is also selected to adapt to the target clearance as chosen. In addition it is provided with some kind of safety margin which could take up some excessive slack without reaching its maximum limit, which in turn would result in failure to meet the expected clamping force.
One major drawback upon brake operation is that the brake pad and the brake disc will notably expand due to the excessive heat generation which could result in that the contact between these components, especially after an immediate consecutive braking, cannot be completely dissolved, leading to dragging and thereby further heat generation and extremely large wear.
For the purpose of reducing the risk of dragging, the adjustment mechanism of whatever design is normally configured to reduce only a certain fraction of the excessive clearance at each brake operation, i.e. the entire excessive clearance will consecutively be reduced by way of several brake operations. In case of some kind of disturbances during the braking process or imperfection in the components, such as tolerance deviations, such selected way of operation further reduces the risk of some kind of over-adjustment and dragging when compared to the compensation of the entire excessive clearance at one single brake operation only.
The torque limiting clutch as used in the adjustment mechanisms according to the prior art could include roller-ramp clutches as e.g. disclosed in WO 2011/113554 A2, or a common friction clutch as used in EP 0 730 107 A2. Further designs such as teethed clutches or torsional springs could be embodied as well.
The adjustment mechanisms as explained above actually act and perform the adjustment in the direction of the clearance reduction only. Such single action principle together with the fractional adjustment and with the selection of a rather large target clearance has proved to work well in the designs and embodiments of the prior art as known.
Nonetheless, such approach seems to be not an optimum solution for achieving the best possible utilization of the available actuator stroke. Namely, providing the desired safety margins to avoid the risk of dragging as described above in turn comes with a large actuator and/or stiff and by that heavy caliper, which generally is undesired both with respect to fuel savings, vehicle handling and manufacturing costs.
Therefore, in addition to the already given adjustment with respect to the reduction of wear-induced clearance there exists a need to provide an adjustment so as to increase the clearance, which proves to be too small with respect to dragging risks, independent from the clearance adjustment to compensate for wear.
From EP 1 524 449 A1 and DE 102 09 567 A1 it has been known to use electric motors as brake adjusters which drive the brake pads towards and away from the brake disc in order to always maintain a predetermined distance between these components, which driving is determined by electronic means. Such design, however, proves to be technically complicated, costly to produce and moreover sensitive to disturbances of all kinds.
From the prior art as mentioned there arises the need to eliminate the different problems and drawbacks associated therewith.